[boost] Re: parameters library and rvalue pitfall

From: Daniel Wallin (dalwan01_at_[hidden])
Date: 2005-05-09 18:04:27

David Abrahams wrote:
> Daniel Wallin <dalwan01_at_[hidden]> writes:
> 
> 
>>David Abrahams wrote:
>>I propose that the user tells index_result<> if the result of the
>>default expression should be by reference, so that:
>>
>>   index_result<.., int>::type whatever = p[x | 0];
>>
>>whatever stores a copy of 0 in the default-case, and stores a reference
>>to p[x] otherwise.
>>
>>   int default_ = ...;
>>   index_result<.., int const&>::type whatever = p[x | default_];
>>
>>See what I'm saying now? Am I missing the point?
> 
> 
> That's not too bad.
> 
> I think we might be able to make it a bit safer by detecting when the
> default is a non-const rvalue and having p[...] 

Can we really detect that?

> return by value in that case. However, that might come at the cost of
> some efficiency if the user also happens to use a non-reference
> variable:
> 
>   index_result<.., heavyweight_type>::type = p[x | rvalue_expr() ]
> 
> Of course, when it's heavy the user will probably be using || for lazy
> defaults, in which case, can we protect her?

There's only so much we can do. This case seems hard to get wrong for
the user.

-- 
Daniel Wallin